The invention relates to a refrigeration method and apparatus employing a compressor refrigeration system and an absorber refrigeration system having separate closed circuits, the refrigerant of the compressor system being used for cooling a refrigerant of the absorber system.
Austrian Pat. Specification No. 192,428 and U.S. Pat. No. 2,781,644 disclose known refrigeration methods and apparatus. In one of these known methods, the compressor refrigerating system acts as an auxiliary refrigerating system which makes it possible to operate the absorber refrigeration system as a whole at a lower temperature level. For this purpose, not only is the condenser of the main refrigerating system cooled by an evaporator of the auxiliary refrigerating system but also a further evaporator, operated by the auxiliary refrigerating system, cools the absorber vessel. The temperature level of the absorber system can in this way be reduced sufficiently so that water at ambient temperature or air at ambient temperature can be used for boiling out the absorber composition. However, the cost of this system in terms of apparatus is relatively high; above all, it serves to allow the attainment of particularly low degrees of refrigeration. In the case of another known system, the evaporator of the compressor refrigerating system is located in the absorber vessel which is disposed downstream of the absorber refrigerating system evaporator. In consequence, it is intended to achieve temperatures down to -70.degree. C.
An arrangement is also known (German Pat. Specification No. 1,215,181), which operates with a compressor refrigerating system and and absorber refrigerating system, the condenser of the compressor refrigerating system being within the evaporator of the absorber refrigerating system. In this case, the absorber system is used for cooling the compressor condenser. The compressor is in this case supplied by an electricity accumulator so that the entire arrangement can be rendered portable and independent of a fixed electric lighting system. The arrangement is to be used in conjunction with a transport container which has to be cooled, whereby, during transport, refrigeration is provided both by the absorber as well as by the compressor refrigerating system. The power required is drawn from the electricity accumulator. The use both of a compressor and also of an absorber system makes it possible to achieve particularly low temperatures. However, a disadvantage with this just-mentioned arrangement is that a power source is nevertheless required which, by virtue of its being constructed as an electricity accumulator, is bulky, very heavy and not very efficient. Also, the power density of an electricity accumulator is very low in relation to its volume as well as its weight.
In the case of a compressor refrigerating system, the power is supplied mechanically via the compressor drive. Usually provided as the compressor drive is an electric motor which draws its power from the mains. In the case of the absorber method, on the other hand, the power is supplied as thermal energy in that the absorber composition is heated, the refrigerating medium being thereby expelled. In this case, heating of the absorber composition and expulsion of the refrigerant on the one hand as well as recovery of the refrigerant when the absorber composition has cooled, on the other, can be effected continuously or in stages. However, a disadvantage of the absorber method is the relatively poor level of efficiency. Also, the number of combinations of materials which can be used as an absorber composition and as a refrigerant is very small. One very expedient combination of materials is for example lithium chloride as the absorber composition and methyl amine (CH.sub.3 -NH.sub.2). Methyl amine has an evaporation heat of some 200 kg.cal/kg and lithium chloride has an absorption capacity of 2 kg methyl amine per kg of lithium chloride. At the conventionally attainable temperatures and pressures in the absorber system, it is necessary to heat the lithium chloride to approximately 200.degree. C which gives rise to a relatively violent gas, so that the spongy-porous structure of the lithium chloride is rapidly destroyed. A relatively high temperature and thus a high pressure is however required in this zone of the system, because in the condenser, for liquefaction, it is generally not possible to cool down farther than approximately ambient temperature. At this relatively high temperature, however, it is essential to go below the boiling point, which is possible only by relatively high pressure. A disadvantage of the compressor method system is that a mechanical driving energy is required which can only be produced easily and conveniently if an electricity supply mains (high quantity electrical energy source) is available. However, if such a mains supply is not available, then the only driving possibility is a cumbersome, expensive and extremely noisy internal combustion engine. Refrigerated transport containers in which heat-sensitive goods such as for example easily spoiled foodstuffs, are transported, are provided with their own refrigerating plant which derives its power either from its own internal combustion engine or from the driving engine of the vehicle. These refrigerating plants are generally constructed as compressor systems.
An object of the present invention is to provide a refrigeration method which achieves satisfactory efficiency and with which it is possible to manage, at least for a time, without an external power supply, so that it is particularly suitable for the refrigeration of transporter tanks.
The present invention contemplates overcoming the above-mentioned disadvantages of previous systems by providing that the compressor refrigerating system and the absorber refrigerating system are alternatingly in operation and the refrigerant store of the absorber system during the period of operation of the compressor system is made up again by boiling out the absorber composition and that for this boiling out the coolant compressed and therefore heated by the compressor is used.
The advantage of a so combined system in accordance with the present invention lies in the extremely improved efficiency since the heat getting free during cooling of the coolant is used for boiling out the refrigerant from the absorber composition. By cooling the refrigerant of the absorber system by means of the coolant of the compressor system a favorable temperature-pressure-level in the absorber system is achieved. This system is especially suitable for the cooling of containers; yet by virtue of its improved level of efficiency it is also quite suitable for a stationary refrigeration wherein the advantages of the method according to the invention also become effective. Since a large part of the energy supplied to the compressor system in the form of heat is getting lost unused, this heat, however, being utilized by the inventive method, it is possible to considerably reduce the costs for energy during operation of a system according to the invention. Moreover, the absence of electric current will not cause damages to the goods to be cooled, since during the period of the absence of current or power supply failure the absorber system can take over the refrigeration.
The method according to the invention is preferably applied for the cooling of containers. Thereby, the compressor system operates during the transporting pauses and during the transport the absorber system operating without power supply and being fed from a refrigerant store is in operation, the refrigerant store of the absorber system being made up again during the transport pauses by boiling out the absorber composition.
A particular advantage of the combined method according to the invention resides in that the compressor refrigeration system can be operated during periods when a suitable power source, for example the public electricity supply mains, is available. During this time, the absorber refrigeration system is "charged." The absorber system can then perform its function for a definite time during which, for want of a suitable power source, the compressor refrigerating system cannot operate. A further advantage of the system lies in the fact that, due to the use of the compressor system for cooling the refrigerant of the absorber system, the operating points of the absorber system can be so located that on the one hand the absorber system acquires an improved level of efficiency because its working temperature differential can be widened and because, on the other hand, by virtue of the displaced working point, a substantially reduced loading on the absorber material is achieved, so that the absorber material has a long effective life and hitherto unsatisfactory absorber materials can be reliably utilized.
For cooling the refrigerant of the absorber system, it is possible for example to use the liquid refrigerating medium of the compressor system or, as is preferable, the gaseous coolant leaving the evaporator of the compressor system can be used for cooling the refrigerant. Thus, the refrigerating capacity of the refrigerant of the compressor system such as it still has after leaving the evaporator, can be used for further cooling of the refrigerant of the absorber system.
The refrigerant can be cooled in various ways. In one preferred method according to the invention, refrigerating medium supplied in gaseous form to a condenser from an absorber vessel provided with absorber composition is subsequently passed through a heat exchanger to a liquid refrigerant collector whereby, in order to cool the refrigerant, gaseous refrigerating medium flowing from the evaporator to the compressor flows through the heat exchanger. In this embodiment, the "used gas" of the compressor system, which has already fulfilled its function of cooling a space, is utilized in order to extract still more heat from the refrigerating medium. In preferred embodiments where the resultant cooling effect is not sufficiently pronounced, it is contemplated by the invention to provide that liquid refrigerating medium be fed to the heat exchanger, bypassing the evaporator, evaporation being undertaken in the heat exchanger. In consequence, a very pronounced refrigerating effect can be achieved. However, it is also contemplated by the invention to use both already evaporated coolant coming from the evaporator of the compressor system as well as liquid coolant derived from the feed to the evaporator of the compressor system, in order to extract sufficient heat from the refrigerating medium in the heat exchanger.
The refrigerating medium of the absorber system rendered liquid and further cooled in the heat exchanger is collected in a collecting vessel from which it can be extracted for the purpose of cooling. Thus, the collecting vessel serves as a refrigerant store which is thermally well-insulated in order to avoid cooling losses. In order, upon prolonged storage of the refrigerating medium, to avoid an undesired rise in the temperature in the collecting vessel, a preferred form of implementation of the method of the invention provides for the refrigerating medium of the compressor system which leaves the heat exchanger to be fed through a cooling coil of the collecting vessel for the refrigerating medium of the absorber system. It will be appreciated that the cooling coil can also be directly connected to the line leading to the evaporator, so that liquid refrigerating medium would be supplied to the cooling coil through a correspondingly dimensioned throttle and vaporize in the cooling coil. Generally, however, it is sufficient to supply to the cooling coil "used gas" from the heat exchanger in order to dissipate again the heat inevitably penetrating the collecting vessel even through good insulation.
In order to "boil out" the absorber composition, the heat from an additional heat source may be used. However, in the case of a particularly preferred embodiment of the invention, the heat of the refrigerating medium compressed by the compressor is utilized for "boiling out" the absorber composition. This procedure has the advantage of substantially enhancing the efficiency of the installation because the heat contained in the compressed refrigerating medium is not uselessly imparted to the ambient but is utilized in order to heat the absorber composition. It will be understood that if the quantity of heat supplied by the compressed refrigerating medium is not sufficient, an auxiliary heat source is used. Once the compressed refrigerating medium has given off a substantial part of its heat to the absorber composition, the refrigerating medium having in consequence been expelled from the absorber composition, the refrigerating medium can in known manner be further cooled and liquefied by a condenser, unless liquefaction has already taken place during heating of the absorber composition.
In the case of a preferred further development of the method according to the invention, prior to commencement of transport, the absorber composition is cooled in the absorber vessel and, to this end, either liquid refrigerating medium is supplied, bypassing an evaporator of the compressor refrigerating system and evaporation is effected in the absorber vessel or gaseous and still relatively cold refrigerating medium leaving the compressor refrigerating system is supplied. The particular advantage hereby resides in the fact that at the commencement of transport the absorber refrigerating system is at once fully capable of functioning. The ability of the absorber composition to accommodate refrigerating medium is greatly dependent upon temperature, which is why the absorber vessel is also heated in order to expel the refrigerating medium from the absorber compostion. Prior to commencement of transport, cooling of the absorber composition makes the full refrigerating efficiency immediately available. Particularly advantageous is the form of the method according to the invention, wherein the gaseous refrigerating medium leaving the evaporator of the compressor refrigerating system is used for such cooling of the absorber composition. This gaseous refrigerating medium, when it leaves the evaporator, is at a temperature below the temperature obtained in the transport container or is at most equal to this temperature. This temperature, generally of considerable minus degrees, is sufficient adequately to cool the absorber composition in the absorber vessel.
The compressor refrigerating system and the absorber refrigerating system are operated with different substances. Preferably, the compressor refrigerating system is operated on a compound which is commercially available as Frigen. Also various combinations of substances are suitable for the absorber refrigerating system.
Methyl amine is the preferred refrigerating medium while lithium chloride is preferably used as the absorber composition.
In the case of a further development of the method according to the invention the absorber composition is additionally cooled in an absorber vessel during the operation of the absorber. By additional cooling through cooling liquid during the absorption, the absorber composition is maintained at a low temperature, so that the pressure level in the absorber circuit may be kept low, thus leading to an improved efficiency of the absorber system. The invention further contemplates providing embodiments where the refrigeration is entirely or partly designed as surface refrigeration of the absorber vessel and the cooling liquid is subsequently watched for a refrigerant contained therein so that it is always immediately ascertainable if a leak occurs in the absorber system. In this way it is possible to prevent that poisonous refrigerants of the absorber system will reach the waste-water canalization together with the cooling liquid.
The invention furthermore relates to an apparatus for economically implementing the above-described method. The present invention contemplates refrigerating apparatus which is provided with a compressor with a drive for compressing a gaseous refrigerating medium with, on the downstream side, a condenser for liquefying the refrigerating medium, from which a line leads to a pressure relieving throttle disposed in a space which is to be cooled and which has adjacent to it an evaporator from which a return line leads to the vacuum connection of the compressor. The apparatus further includes an absorber refrigerating system containing a refrigerating medium and provided with an absorber composition in an absorber vessel from which a gas line leads to an absorber condenser, whence a line leads to a liquid refrigerant collecting vessel, connected to which there is, disposed in the space to be refrigerated, a pressure relieving element with an adjacent evaporator, the return line from which discharges into the absorber vessel.
A refrigerating apparatus is generally known (DAS1,215,181) utilizing both compressor and absorber systems. As already discussed above, a disadvantage of this prior apparatus is that the absorber refrigerating system serves only to cool the compressor condenser while the compressor is supplied by an electricity accumulator in order to render the entire arrangement portable and independent of the electric lighting system. The carrying of electric power makes the arrangement bulky, very heavy and not very efficient.
An object of the present invention further resides in the provision of a refrigerating apparatus for carrying out the method according to the invention and which differs by special features and advantageously from the known refrigerating apparatus. On the basis of a refrigerating apparatus of the type described at the outset, this problem is, according to the invention, resolved in that between the absorber condenser and the liquid refrigerant collector there is a heat exchanger preceding the vacuum (or low pressure side) connection of the compressor so that for cooling the refrigerant of the absorber system flowing through the apparatus, refrigerating medium of the compressor system flows through the heat exchanger.
One advantage of the refrigerating apparatus according to the invention resides in the fact that satisfactory efficiency is achieved and that the two refrigerating circuits can be operated individually or jointly, as desired. A particular advantage of this refrigerating apparatus which makes it pre-eminently suited for refrigerating transport containers, lies in the fact that it can operate for certain periods of time to generate cooling of a space without the supply of mechanical or electrical power. A further advantage of the apparatus according to the invention lies in the fact that the absorber system can be designed in a way which not only provides for greater efficiency of this system but also opens up an opportunity for the use of combinations of materials which hitherto not readily usable for the absorber composition and the refrigerating medium. So long as mechanical or electrical energy is available, the space to be cooled is cooled by the compressor system; at the same time, the absorber system is being "charged," so that the absorber system can take over the work of cooling during those periods when the compressor system cannot be utilized for want of driving power. As a result, refrigerating chambers or transporters containers which are fitted with refrigerating equipment according to the invention can be kept permanently cool. For example, it is possible in this way to avoid the temperature in a stationary refrigerating chamber rising excessively on power mains failure and also to ensure that portable refrigerating chambers remain cooled in transit or need not be provided with a combustion engine to drive the compressor refrigerating system, which would be uneconomical and expensive and which would moreover entail disturbance, noise and smell.
In a preferred embodiment of the invention, a cooling coil is disposed between the heat exchanger and the suction connection of the compressor in a vessel for collecting liquid refrigerant of the absorber system. As a result, the refrigerant present in the collecting vessel retains the lower temperature achieved and, despite the heat insulation provided, does not become undesirably heated. It will be appreciated that this cooling coil can also be incorporated upstream of the heat exchanger, in other words between the compressor system evaporator and the heat exchanger.
The cooling medium feed connection of the heat exchanger is made in various ways in accordance with various preferred embodiments of the present invention. In one preferred embodiment of the invention, the feed connection is made downstream of the outlet from the evaporator of the compressor system. In this embodiment, it is the "used gas" flowing to the compressor from the evaporator disposed in the space which is to be refrigerated which is used for cooling the refrigerant of the absorber system.
With other embodiments of the invention, on the other hand, a feed connection of the heat exchanger for refrigerant is made to the outlet side of a pressure relieving element, to which liquid refrigerating medium for the compressor system is supplied so that the heat exchanger serves as an evaporator for the refrigerating medium. This embodiment is used when a very marked cooling of the refrigerating medium of the absorber system is required and the gaseous refrigerant leaving the evaporator of the compressor system cannot generate an adequate cooling effect. The heat exchanger is preferably avoided with two feed connections for the refrigerating medium which can be used simultaneously or optionally. In other preferred arrangements, the heat exchanger is provided with only one such feed connection, the supply line of which can be changed over as required. In a further development, a constant or intermittent change over can be provided for the heat exchanger feed, controlled preferably by a thermostat. By virtue of this reversibility, according to the refrigerating output desired or according to the given ambient conditions, the desired operating pattern of the apparatus can be achieved.
The thermostat can be controlled as a function of various factors. In the case of preferred embodiments of the invention, the thermostat is sensitive to the temperature of the coolant (of the compressor systems) emerging from the heat exchanger or of the refrigerant (of the absorber system) emerging or the temperature of the refrigerant in the collector which becomes the controlled variable. According to the choice of the controlled variable, so the feed to the heat exchanger can be variously controlled.
In a further development of the present invention, a non-return valve is incorporated into the line between the absorber vessel and the absorber condenser to prevent refrigerant flowing in the direction of the absorber vessel.
The efficiency of the known refrigerating apparatuses is relatively low. A considerable improvement of the already satisfactory efficiency of the apparatus according to the invention is, in the case of a particularly preferred embodiment, achieved in that a condenser provided downstream of the pressure connection of the compressor is provided and is constructed as a heat source for the absorber vessel. The compressed coolant leaving the compressor in a gaseous form is heated by the compression process. This heat is usually given off to the ambient air in a subsequent condenser. In the case of the refrigerating apparatus according to the invention, however, this heat is not uselessly dissipated to the ambient air but is used to heat the absorber composition in the absorber vessel. Condensation of the coolant can then take place within the absorber vessel. However, if the temperature of the coolant, when leaving the absorber vessel, is still above boiling point, then a conventional condenser is provided on the downstream side to extract the residual heat from the coolant and discharge it into the air passing by under the stimulus of a fan. The heating condenser which serves as a heat source can be provided instead of or in addition to a conventional heat source. In a preferred form of embodiment of the invention, the heating condenser is constructed as a heat exchanger built into the absorber vessel. Likewise, the heating condenser could also be constructed as a heating coil surrounding the absorber vessel.
In a preferred embodiment of the invention, the absorber composition is held in the absorber vessel on filters disposed one above another and a heat exchanger built into the absorber vessel is constructed as a central spiral tube which is extended through the annular filters. In this embodiment, the filters are preferably mechanically connected both to the spiral tube and also to the outside wall of the absorber vessel in a readily heat-conductive manner. Location of the absorber composition on filters ensures a good long-term activity for the absorber composition, because the active surface of the absorber composition is very large. The absorber composition in each layer is easily accessible to the gaseous refrigerating medium both from above and also, by virtue of the filter structure, from below. The readily heat-conductive connection of the filters to the spiral tube can ensure rapid supply or dissipation of heat.
In a preferred further embodiment of the invention, the spiral tube is incorporated between the pressure connection of the compressor and the condenser of the compressor refrigerating system or between the condenser of the compressor refrigerating system and/or the evaporator of the compressor refrigerating system and the suction connection of the compressor. Therefore, in the case of such an apparatus, the spiral tube is usable for boiling out and thus for expelling the refrigerant during pauses in transport, utilizing the waste heat from the compressor which must in any event be dispersed. In addition, the spiral tube is also usable for cooling the absorber composition prior to commencement of transport, in order to cool the absorber vessel with the absorber composition by using excess refrigerating output from the compressor system and thus ensure full efficiency of the absorber system from the time transport commences. Generally, long prior to commencement of transport, the container to be cooled is cooled to such an extent that the compressor refrigerating system only needs to cover refrigerating losses and therefore its full efficiency is no longer required. This excess performance can be utilized to cool the absorber vessel with the absorber composition.
If the spiral tube is used both for heating and also for cooling the absorber vessel, then by reason of the differing conditions, the heating or cooling efficiency may be too low or too high. In a further preferred embodiment of the invention, therefore, the spiral tube is used only for heating the absorber vessel and incorporated into the absorber vessel is a cooling coil which can be incorporated between the suction connection of the compressor and the evaporator of the compressor refrigerating system or the condenser of the compressor refrigerating system. The cooling coil can thereby switched on and off via a controlled valve, for example a time-dependently controlled valve. In preferred embodiments of the invention, a multi-way valve is provided to switch over the heating or cooling circuits of the absorber vessel and the multi-way valve or valves can be switched over as a function of time according to the pressure obtaining in the absorber vessel.
In the absorber vessel, as already mentioned, the absorber composition is held on filters in order to achieve a large active surface and in order to ensure a rapid exchange of refrigerating medium. In order also to achieve rapid heating of the absorber composition, in a preferred embodiment of the invention, the filters are connected in a readily heat-conductive manner to the spiral tube; furthermore, the filters are mounted on a central carrier so that they can be removed from and replaced in the absorber vessel together with the carrier and the absorber composition. This embodiment of the invention not only provides for rapid heating of the absorber composition and thus rapid expulsion of the refrigerant; in addition, the fact that the carrier, filters and also preferably the spiral tube are constructed as one unit means that the absorber composition can be rapidly and easily replaced when necessary.
In a preferred further development of the present invention, a refrigerating or cooling apparatus is additionally incorporated and the filters are connected thereto in a readily heat-conductive manner. In the case of this embodiment, both the heating coil as well as the refrigerating apparatus can always be adapted in optimum fashion to the working conditions, and, on the one hand, a rapid but nevertheless gentle boiling out of the absorber composition is achieved and also, as prior to commencement of transport the absorber composition can be cooled in a minimum of time. As a result, even with short operating times, recharging of the absorber circuit can be achieved and rapid readiness for operation at full efficiency of the absorber circuit can be attained at the commencement of the next transport stage. The advantages of the refrigerating method according to the invention and of the refrigerating apparatus according to the invention are experienced particularly when they are applied to a transportable refrigerating container. In the case of a preferred embodiment, therefore, a refrigerating apparatus according to the invention is combined with a transportable refrigerating container, the compressor refrigerating part operating during pauses in transport and charging the absorber refrigerating part, the absorber refrigerating part taking over the refrigeration during transport. During pauses in transport, the compressor refrigerating part cools the refrigerating container or maintains it in a cold state and also the collecting vessel is filled with refrigerant and if necessary also the absorber vessel is cooled prior to commencement of transport. In this case, the compressor refrigerating part is designed according to the length of the anticipated pauses in transport, in terms of its efficiency, to ensure that, at all times, there is an adequate quantity of refrigerant for the period of transport present in liquid form in the collecting vessel, and to ensure moreover that the temperature in the refrigerating container never exceeds the limit value required.
In the case of a preferred embodiment of a refrigerating apparatus the absorber vessel is provided with additional cooling device being connected to a cooling liquid store. With the additional refrigeration not only the absorption process is favorably influenced and the maintenance of a low pressure level is made possible, but also the efficiency of the absorber system will be improved. Thereby, a circuit is preferably provided for the cooling liquid.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.